The present invention relates to data communication systems, and more particularly to an arrangement for permitting a plurality of processor units in a communications network to communicate among one another on a single serial bus.
In one common type of communications network, a number of processor units are connected to a single serial data communications bus which comprises any one or a plurality of transmitting media, such as coaxial cable, optical fiber, or others. The processor units connected to the serial bus will hereinafter be designated as stations.
Any particular station may comprise a variety of hardware elements and may be dedicated to various purposes. For example, a single network may comprise stations dedicated to a specific control or monitor task, stations dedicated to data accumulation or analysis, and stations dedicated to peripheral equipment for user input and output. Each station has in common a transmitter-receiver unit which enables the station to communicate with any of the other stations connected to the serial bus.
An important characteristic of this typical communication network is that no dedicated station is provided for controlling the serial bus communications. As a result, communications in this network are not crippled by the failure of a single station. There remains, however, the problem of arbitrating orderly access to the single serial bus among the plurality of connected stations.
Copending application, Ser. No. 210,700, is addressed to the problem of arbitrating orderly access to the serial bus and presents an improved bus access arrangement. In this arrangement, each station in the network is provided a discrete time window during which it may initiate transmission on the single serial bus. If access is not claimed within the station's time window, the station is prevented from issuing the transmission until its time window reappears during the next round of bus arbitration. In this arrangement, each station is given a single opportunity to issue a transmission before any one station has the opportunity to make a second transmission. This is termed a democratic access arrangement. This prior art scheme operates efficiently to prevent domination of the serial bus by a few stations with high priority by allowing each station the opportunity to transmit within a reasonable period of time. Problems may arise, however, where stations in the network sometimes require immediate access to the serial bus to transmit an emergency message.
In the practical application of a communications network, the need sometimes arises for a network station to communicate immediately with one or more other stations within that network. Where such a network comprises a relatively large number of stations, the prior art scheme revealed by the aforestated copending application may be unable to provide access to the serial bus quickly enough to accommodate the emergency message. Such would be the case where a station needing to issue an emergency transmission must await the completion of transmissions by a number of stations concurrently awaiting channel access for possibly less urgent transmissions.
Hence, it would be advantageous to develop a bus access arrangement which, in addition to providing for orderly and collision-free access by a plurality of stations, also permits relatively quick access to the bus by any station in the network for transmission of an emergency message.